THE NEW WORLD MONKEYS are quantitative differences, the Cebidae having an additional molar tooth on each side above and below. In the Cebidse the upper central incisors have chisel edges ; they are larger than the upper lateral incisors, whose crowns are conical in the unworn condition. The lower incisors are slightly procumbent. The canine teeth are large and the lower ones pass up into diastemata between the upper incisors and canines.^ The incisor and canine teeth are not so specialized as those of the Lemuroidea. The three upper premolar teeth have two cusps lying antero-posteriorly, and they decrease in size in the order 1, 2, 3. The first lower premolar tooth is larger than the second and third, which are almost equaL It has one large and one small cusp; and the second and third lower premolars have more equal external and internal cusps. Gregory (507) points out that the three premolar teeth are remnants of an ancestral heritage of three teeth, and the number distinguishes the Platyrrhini from the Catarrhini. Before proceeding to the consideration of the molar teeth it is necessary to refer to the terminology which is employed in the description of their cusps. The upper molar teeth have four cusps—two anterior and two posterior—and the lower teeth have a small postero-median cusp in addition. These cusps or tubercles have been named as follows (they are shown in fig. 11) ;— \ Upper Molar Teeth— Antero-external cusp .........paracone U?A). Antero-internal cusp .........protocone (PR). Postero-external cusp.........metacone (M). Postero-internal cusp .........liypocone (HY).